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EXPERIMENTAL STUDY OF THE CURRENT
FILAMENTATION INSTABILITY
by
Brian Allen
A Dissertation Presented to the
FACULTY OF THE USC GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY
(ELECTRICAL ENGINEERING)
December 2012
Copyright 2012 Brian Allen

Plasma instabilities can produce strong anisotropies, accelerate particles to high energies and generate large electric and magnetic fields. The Current Filamentation Instability, CFI, is of central importance for the propagation of relativistic electron beams in plasmas. CFI has potential relevance to astrophysics (afterglow of gamma ray bursts), inertial confinement fusion (energy transport in the fast-ignitor concept) and beam-driven plasma-based accelerators (placing an upper limit on the plasma density and accelerating gradient). ❧ The work in this dissertation combines a review of theory, particle-in-cell simulations and design and execution of an experiment that resulted in the first conclusive observation of the current filamentation instability. Current theory of CFI is reviewed and discussed. Simulations, with a particle-in-cell code, were conducted to validate theory for the finite size bunch and plasma used in the work and provide insight for the experiment. The design of a high-magnification imaging system to observe the result of the instability and the setup of the experiment are detailed. Finally, the experimental results are presented and discussed. ❧ The experiment was conducted at the Accelerator Test Facility at Brookhaven National Laboratory with the 60MeV e⁻ bunch and 2cm long plasma with variable density. The experiment included the systematic study and characterization of the instability as a function of the beam charge and plasma density, or ratio of bunch transverse size to plasma skin depth c/ɷpe or 1/kpe. The transverse beam profile and size (σᵣ) is measured directly at the plasma exit using optical transition radiation from a thin gold coated silicon window. Experimental results show the transition from plasma focusing (kpσᵣ << 1) to CFI (kpσᵣ >> 1) is characterized by the appearance of multiple (1-5) beam filaments and the linear scaling of the transverse filaments size with the plasma skin depth. Suppression of the instability is demonstrated when lowering the growth rate of the instability by reducing the beam charge. The experimental results are in excellent agreement with theory and simulations.

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EXPERIMENTAL STUDY OF THE CURRENT
FILAMENTATION INSTABILITY
by
Brian Allen
A Dissertation Presented to the
FACULTY OF THE USC GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY
(ELECTRICAL ENGINEERING)
December 2012
Copyright 2012 Brian Allen